As refers to something or someone's appearance or function. Consider the following examples: 'Before I became a teacher I worked as a waiter.' 'I'm going to the fancy dress party as Superman.' 'The sea can be used as a source of energy.' Like has the meaning 'similar to' and is used when comparing things. Look at these examples: 'I’ve been working like a dog.'

Excellent question. To add a little to Robert's helpful answer. "As" can be used for different purposes. Concrete comparisons such as Robert gives. Also may be a 'metaphor' -using a different image to describe something. The 'good boy' might be 'as good as gold'. The gentle hand might touch you 'as lightly as a butterfly" "Like" is used with a 'simile' - see 'simile' is like 'similar', so it's a bit like a metaphor but different: "He ran like the wind, and as soon as he came to the house, he told them the good news, just as it had been told to him in the town. They hugged him like a brother. They hugged him as it he were a brother." Also 'like' can be used to speak of things that are 'likely [ = probable] "It looked like it was going to rain." "It looked like rain was coming." "It looked like the driver would go to sleep, so as soon as possible they stopped for a rest."